This invention is directed to quantum dot light emitting diodes, and in particular, the quantum dot nanoparticles forming the emitting layer of the quantum dot light emitting diode.
Light emitting diodes (LEDs) have become increasingly used both commercially and in the home. Because of their low energy consumption, small size and long lifetime LEDs have become extremely popular as a result of the desire for reducing carbon footprint.
One type of light emitting diode is the quantum dot light emitting diodes (QD-LEDs). Quantum dots (QDs) are semiconductor nanocrystallites having radii approximately equal to or smaller than the bulk exciton Bohr radius of that semiconductor. Quantum confinement of electrons and holes leads to an increase in the band gap of the QDs as the size approaches or becomes less than the Bohr radii. When the band gap increases, the optical absorption of and emission from quantum dots shifts to higher energies as the size of the dots decreases.
As seen in FIG. 1, QD-LEDs are known in the art and employ neat layers and/or composite layers of materials for efficient charge injection, charge transport and charge recombination for radiative emission, as known from PCT/US2010/041208. A QD-LED, generally indicated as 10, may include a layer of Zn0 nanoparticles which function as the electron transport and injection layer 14. A cathode 12, preferably formed of aluminum, injects electrons into the ZnO nanoparticle layer 14. Electrons. pass through the electron transport/injection layer 10 and are then injected from the electron layer 10 into a QD light emitting layer 16. The QDs typically have a core of CdSe and a shell of ZnS. The light emitting layer 16 is supported by a hole injection and transport layer formed as a TFB layer 18 and PEDOT:PSS layer 19. The hole injection and transport layer is in turn supported by an indium tin oxide anode 22 from which holes are injected, which in turn is supported on a solid glass substrate 24. The hole injection and transport layer may also be formed using Ni0 or Mo03 nanoparticles. Cathode 12 is electrically connected to anode 22 across a voltage source 30. In this way, the QD-LED is excited 8; to emit light through glass substrate 24.
The prior art QD-LED has been satisfactory. However, because of inappropriate compositions and discontinuities in composition in the QDs of the light emitting layer, the known QD-LEDs are not bright, and are inefficient requiring more energy for less luminance at the designed wavelength.
Accordingly, a QD for a QD-LED which overcomes the shortcomings of the prior art is desired.